This study has two specific aims. The first is to compare the effects of imipramine, diazepam, and placebo on the panic attacks experienced by panic disorder patients. This study will compare diazepam to a standard treatment (imipramine) and placebo. The effect of benzodiazepines on panic attacks is controversial, and diazepam has never been tested in a placebo controlled trial. The second specific aim is to compare the effects of imipramine, diazepam, and placebo on lactate-induced panic anxiety in panic disorder patients. This is a test of the validity of lactate infusions as a model of panic anxiety; treatments that are effective for panic should effectively decrease lactate-induced anxiety, and treatments that are ineffective should have little or no effect. The effect of imipramine on lactate-induced panic anxiety has never been tested in a controlled study that takes into account the biasing effects of patient selection, placebo treatment effects, and desensitization to the infusion procedure. The effect of diazepam treatment on lactate-induced anxiety is unknown. Patients with panic attacks will receive separate infusions of sodium lactate and dextrose in random order under double-blind conditions and have their symptoms rated. Patients will then be randomly assigned to treatment with imipramine, diazepam, or placebo. Panic attacks will serve as the target symptom, and assessment measures will be done weekly. All patients, irrespective of treatment response, will be reinfused with lactate and dextrose after eight weeks of treatment and have their symptoms rated again. If findings from lactate-induced panic attacks are generalized to naturally occurring panic attacks, treatment response to imipramine, diazepam and placebo should predict response to lactate infusions. This finding would also suggest that pre- and post-treatment lactate infusions. This finding would also suggest that pre- and post-treatment lactate infusions can be used to test new drug treatments for panic.